We take as a given that fat is bad and thin is good. But what if that’s not the case? In her new book, “Body of Truth,” Harriet Brown says studies have shown that being overweight does not result in a shorter life span, and that dieting — especially among children — leads to people becoming fatter and more unhealthy. An excerpt:

Every year in January — the height of our annual weight-loss frenzy — 50 million Americans go on a diet. That’s roughly one-sixth of the entire population.

“Body of Truth: How Science, History and Culture Drive Our Obsession with Weight — and What We Can Do About It” by Harriet Brown (Da Capo Press)

Many of those dieters give up after about two weeks but try again and again over the course of the year, which means, estimating conservatively, that the average 45-year-old American woman has been on 50 diets in her adult life.

I was one of those miserable dieters for many years, starting at 15, when my mother and I joined Weight Watchers together, each with the goal of losing 20 pounds. I wasn’t the only teenager dieting, of course; in the 1970s, the average age for girls to start dieting was 14. (Now, by contrast, it’s 8 years old.)

In 2013, Americans spent more than $60 billion on weight-loss products, and that number keeps rising. A lot of bottom lines are at stake in the efforts to trim our bottoms.

Unfortunately, the evidence suggests that dieting makes people neither thinner nor healthier. Quite the opposite, actually: Nearly everyone who diets winds up heavier in the long run, and many people’s health suffers rather than improves, especially over time. Repeated dieting in particular causes a cascade of negative physical and psychological consequences.

Dieting can make people thinner for a while — six months, a year or two, maybe three. Which, coincidentally, is about how long most studies follow dieters and how they claim success.

In reality, your chance of maintaining a significant weight loss for five years or more is about the same as your chance of surviving metastatic lung cancer: 5%.

It doesn’t matter what flavor of diet you try — Paleo, Atkins, raw, vegan, high-carb, low-carb, grapefruit, Ayds (remember those chewy chemical-infused caramels?) — only 3% to 5% of dieters who lose a significant amount of weight keep it off. Weight-loss treatments are cash cows, in part because they don’t work; there’s always a built-in base of repeat customers.

‘Set point’ vs. starving

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The mind-boggling element here is that we’ve known diets don’t work for a long time, and so has the medical establishment.

Back in 1958, A.J. Stunkard, a well-known obesity researcher and professor at the University of Pennsylvania, wrote, “Of [obese] people who lose weight, most will regain it.”

In the 1970s and 1980s, scientists like Paul Ernsberger, now a nutrition professor and researcher at Case Western University, began to document the ways dieting did (and usually didn’t) work.

Nearly 30 years ago, Ernsberger and a colleague published an exhaustive review of the links between health and obesity.

They pointed out that 16 long-term international studies had found that overweight and obesity were not major risk factors for death or heart disease; a US panel on obesity had relied heavily on data from the insurance industry, and since fewer heavy people bought life insurance (because they had to pay more for it), the mortality rates linked to obesity skewed higher.

Ernsberger also drew attention to the fact that mortality rates were lowest in the overweight category on the BMI chart. He and his colleague hypothesized that some of the conditions associated with obesity, like hypertension and elevated cardiovascular risk, actually came from failed treatments — that is, weight cycling, or losing and regaining weight over and over.

And they suggested that many doctors’ disapproval of obesity was based on “moral and aesthetic biases” rather than medical facts, a suggestion that has since been borne out by research into doctors’ clear prejudices around obesity.

While the ultimate effects of dieting aren’t news, we do know more these days about the process, pros, and cons of dieting than we did 40 years ago. We know, for instance, that dieting nearly always makes people heavier over time. In one study of Finnish twins, the more diets people went on, the higher their risk of becoming overweight and the faster they gained weight later in life.

In one study, Polivy attached little baskets of Froot Loops above the cages of food-deprived rats, so the animals could see and smell the Froot Loops but couldn’t get to them. Those rats showed much higher levels of stress hormones than the rats who weren’t tormented by the Froot Loops.

When they were allowed to eat as much as they wanted, the first group ate without restraint, gaining significant amounts of weight, while the second did not.

The takeaway, says Polivy, is clear. “Unless you lock yourself in a room, have minimal food brought to you, don’t watch TV, and don’t have access to anything outside, you’ll be surrounded by food cues,” she explains. “Which ultimately take their toll. As soon as you can get food, you eat more and gain back everything and then some.”

There’s a theory that each of our bodies is geared to function best within a certain fairly limited weight range, usually 10 or 20 pounds, called a “set point.” And our individual set points can vary wildly.

A friend who’s the same height as I, and who probably weighs 50 pounds less, gained 10 pounds recently from taking a short-term medication. She knew she’d finish the medication and her weight would return to its usual level. She’s not particularly biased against fat or fat people, never diets, and is very physically active. But the extra 10 pounds made her excruciatingly uncomfortable, physically and mentally. She couldn’t wait to get off the meds so her body could revert to its set point. Which, in good time, it did.

Set point theory suggests it’s relatively easy to lose or gain weight within your range, but a lot harder to go outside it in either direction.

Messing with kids

In 2002, William Klish, a pediatric gastroenterologist featured in the movie “Super Size Me,” told a reporter from the Houston Chronicle, “If we don’t get this [obesity] epidemic in check, for the first time in this century children will be looking forward to a shorter life expectancy than their parents.”

As Klish later admitted, he had absolutely no evidence for this frightening scenario; it was based on his “intuition.” Which didn’t stop it from being replayed in the media and cited by researchers.

So Klish’s doomsday scenario lives on in part because it’s frightening. And fear (whether justified or not) is a big part of the way we talk about weight — especially when it comes to children.

According to the most recent numbers from the CDC, almost half of American kids and teens fall into either the overweight or obese category on the BMI chart.

The problem with BMI is that it’s not an accurate measurement or predictor of health, especially for people who are shorter or more muscular than average. It doesn’t take into account the amount of muscle or fat in a body, or how strong (i.e., heavy) a person’s bones are.

It says nothing about a person’s future risk of disease or death, and it was never intended to. Its creator, Belgian mathematician Adolphe Quetelet, came up with BMI in the 1830s as a way of looking at trends in populations, not in people. But in the late 1970s, researchers began using it precisely as Quetelet hadn’t intended, to categorize individuals’ weight and health status.

It’s been the medical model go-to ever since.

Meanwhile, the cutoff points for BMI have been moved somewhat arbitrarily within the last 15 years.

Starting in 1994, the National Institutes of Health considered children whose BMI put them in the 95th percentile or above for their age overweight; those in the 85th to 95th percentiles were labeled “at risk of overweight.”

In 2005, the categories shifted; now kids above the 95th percentile are labeled “obese,” while those in the 85th to 95th are “overweight.”

And, according to University of Colorado law professor Paul Campos, author of “The Obesity Myth,” another little-known change happened at the same time: Those percentiles were defined using data from the 1960s and 1970s rather than data on kids today, who are both taller and heavier than kids back then.

In other words, writes Campos, “When Michelle Obama claims a third of our children are too fat, what she’s really saying is that what was the 85th percentile on the height-weight charts 40 years ago is about the 67th percentile today.”

The changing definitions make it hard to track how kids’ weights (rather than their weight categories) have actually changed. Like adults, kids’ average weights rose between 1980 and 2000, when they more or less stabilized.

Childhood obesity is commonly referred to as both an epidemic and a call to action: In 2010, when First Lady Michelle Obama launched her Let’s Move! campaign, her goal was “to solve the challenge of childhood obesity within a generation.”

Dianne Neumark-Sztainer, a professor of public health and epidemiology at the University of Minnesota, has studied weight, dieting and health in children and adolescents since the 1990s, as the principal investigator for a series of ongoing studies known as Project EAT.

She’s found that the younger kids are when they start to diet, the heavier they tend to become and the higher their chance of developing risky behaviors like purging, abusing laxatives, bingeing, and overexercising.

In fact, children and teens who diet are significantly heavier 10 years later than those who don’t — even if they weren’t fat to begin with.

Crisis with no proof

Are we really raising a generation that, for the first time, will not live as long as their parents?

Katherine Flegal, an epidemiologist at the CDC’s National Center for Health Statistics, set out to map the relationship between BMI categories and mortality. They expected to find a linear relationship: The higher a person’s BMI, the greater his or her risk of dying prematurely.

But that’s not what they found. Instead, Flegal and her colleagues discovered what statisticians call a U-shaped curve, with the bottom of the curve — the lowest risk of death — falling around 25 to 26 on the BMI chart, making the risk of early death lowest for those now labeled overweight.

People considered “mildly obese” had roughly the same risk of dying as those in the “normal” category. Death rates went up for those on either end of the scale — underweight and severely obese — but not by much.

As soon as Flegal’s analysis appeared in the Journal of the American Medical Association, the excrement hit the air conditioning.

Other researchers claimed her work was shoddy, that she’d left out important data. They said there was no way her results could be accurate. S. Jay Olshansky, a professor of epidemiology at the University of Illinois-Chicago, responded with a journal article arguing that rising obesity rates would shorten lives by two to five years.

That statistic got a lot of attention and is still floating around today, despite the fact that it’s been utterly and thoroughly debunked.

As one of Olshansky’s coauthors, University of Alabama-Birmingham biostatistician David B. Allison, told a reporter from Scientific American, “These are just back-of-the-envelope plausible scenarios. We never meant for them to be portrayed as precise.”

But reports like these, which are grounded in opinion and clear agendas rather than fact, feed an increasingly hostile and confusing public conversation around weight.

At the very least, we’re being taught that dieting is benign, something that can’t hurt us even if it doesn’t really help. The truth is, dieting is actually harmful for many of us for all sorts of reasons. And it doesn’t make most of us thinner or healthier.